Timing and mechanisms of surface and intermediate water circulation changes in the Nordic Seas over the last 10,000 cal years: a view from the North Iceland shelf
Introduction
The North Iceland shelf has recently been the subject of intensive marine geological investigations which showed that this restricted area encapsulated, throughout the last deglaciation and Holocene periods, oceanographic and atmospheric variability that occurred over a much broader area (Andrews et al., 2000; Jennings et al., 2002; Andrews and Giraudeau, 2003; Andersen et al., 2004). Historical, as well as instrumental records have highlighted the sensitivity of this domain to recent oceanic and atmospheric anomalies such as the Great Salinity Anomaly (GSA) in the late 1960s, or the Little Ice Age (LIA) cold spell dated off northern Iceland at 750–100 cal yr BP (Knudsen and Eiriksson, 2002). Both events shared common features off northern Iceland including surface water cooling and freshening associated with increased influence of arctic and polar waters (Dickson et al., 1988; Olafsson, 1999; Eiriksson et al., 2000). Those surface water changes have a profound effect on both primary productivity and sea-ice extent as evidenced by hydrographic survey conducted in these waters over the last 50 yr (Thordardottir, 1984). In addition, it has been shown that surface water changes on the North Iceland shelf are tightly associated with altered overturning of deep and intermediate waters in the Iceland and Greenland seas (Malmberg and Jonsson, 1997), as well as with variations in the flux of Atlantic waters entering the Norwegian Sea (Blindheim et al., 2000). Atmospheric forcing, through variability in strength of the wind stress curl over the Nordic Seas (Jonsson, 1992), as well as changes in the state of the atmospheric pressure system over the North Atlantic (Dawson et al., 2003; Blindheim et al., 2000), are seen as the main drivers of changes in the water-mass structure of the northern North Atlantic.
The North Iceland shelf bears essential components of the present surface and deep circulation of the northern North Atlantic (Fig. 1; Hopkins, 1991; Stefansson, 1962). It is located close to the Arctic Front which separates Arctic/Polar water masses carried by the south-eastward flowing East Iceland Current (EIC) from the North Iceland Irminger Current (NIIC), a branch of the Irminger Current which rounds the western side of Iceland and feeds the North Iceland shelf with warm Atlantic waters. The NIIC reaches the inner part of the North Iceland shelf while the deeper realms are occupied by Arctic Intermediate waters (AIW), down to approximately 500 m, which is formed by convection in the Iceland and Greenland seas (Rytter et al., 2002; Malmberg and Jonsson, 1997). Starting from the mid-shelf area, Atlantic Water carried in the NIIC is submerged beneath colder, fresher Arctic Water carried by the EIC (Fig. 1). This water-column stratification is well displayed in Hunafloaall, where the wedge of Atlantic Water between 100 and 350–400 m water depth is cooled to 3.5 °C and freshened to 34.9‰. This submerged vestige of the NIIC is termed Irminger Intermediate Water (IIW). Beneath the IIW, temperatures decline with depth from 3.5 °C to slightly lower than 0 °C in the deepest parts of the northern shelf troughs. Salinities are 34.8‰, less than those in the IIW. The bottom water mass represents upper Arctic Intermediate Water (Swift, 1986), although the deepest parts of the northern troughs may contain Norwegian Sea Deep Water (Rytter et al., 2002).
We investigate the water-column dynamics off NW Iceland throughout the last 10 000 cal yr using the combined records of coccoliths, proxies for surface water conditions, and benthic foraminifera as tracer of intermediate and bottom water masses in the nearby Nordic Seas. Our higher time resolution proxy in the present study, coccoliths, has been successfully tested in recent works dealing with the Holocene short and long-term evolution of the North Atlantic Drift south of Iceland (Giraudeau et al., 2000), and of its NIIC branch over the inner North Iceland shelf (Andrews and Giraudeau, 2003). It complements another surface water proxy, diatoms, which have been recently investigated in MD99-2269 (Andersen et al., 2004). Recent works on the modern and fossil distribution of benthic foraminifera around Iceland highlighted the close correspondence of species distribution with bottom water-masses and their characteristics (Rytter et al., 2002; Jennings et al., 2002; Jennings et al., 2004).
Section snippets
Material, core chronology and methods
Giant piston core MD99-2269 (66° 37′N–20° 51′W) was collected as part of the IMAGES V cruise of RV Marion Dufresne. The core was retrieved at 365 m water depth from a 30 m thick sediment unit on the floor of Hunafloaall, a north–south orientated depression off N/NW Iceland. Previous works have shown that this core contains a continuous Holocene sedimentary series which accumulated at a rate close to 2 m/kyr (Andrews et al., 2003a). The construction of the shelf sediment unit and the recorded excess
Coccolith species distribution and coccolith carbonate
Coccolith species diversity is typically low as expected for this arctic/subarctic setting (Baumann et al., 2000; Andrews and Giraudeau, 2003). Dominance is equally shared between Coccolithus pelagicus, the cold-end member of the coccolithophore community in North Atlantic waters, and the ubiquitous Emiliania huxleyi (Fig. 2) which is presently responsible for extensive blooms in transitional/subarctic waters as well as in shallow settings along Norway and NE America (Brown and Yoder, 1994).
Discussion and conclusions
The combined coccolith and benthic foraminifera Holocene records are indicative of a clear antagonism in physical–chemical status and sources of water masses between the surface and bottom layers of the water column. In the following discussion, we interpret the Holocene evolution of water-column structure over the North Iceland shelf as a direct consequences of coupled changes in outflow of Arctic bottom and surface waters from the Nordic Seas and inflow of Atlantic waters around western
Acknowledgements
Core MD99-2269 was recovered in 1999 as part of the GINNA/IMAGES V, Leg 3 cruise of the RV Marion Dufresne. We gratefully acknowledge Yvon Balut (IPEV) and J.-L. Turon (DGO, chief scientist) for the outstanding coring operations. The cruise was supported by the French Polar Institute (IPEV) and USA participation was funded through grant NSF-OCE98-09001 and by NSF-OPP-0004233. This paper is also a contribution to an Earth System History (ESH) grant from NSF ATM-0317832. We thank D. Ostermann
References (56)
- et al.
Multi-proxy records showing significant Holocene environmental variabilitythe inner N. Iceland shelf (Hunafloi)
Quaternary Science Reviews
(2003) - et al.
The N and W Iceland Shelfinsights into Last Glacial Maximum ice extent and deglaciation based on acoustic stratigraphy and basal radiocarbon AMS dates
Quaternary Science Reviews
(2000) - et al.
Decadal to millennial-scale periodicities in North Iceland shelf sediments over the last 12 000 cal yrlong-term North Atlantic oceanographic variability and solar forcing
Earth and Planetary Science Letters
(2003) - et al.
History of the last deglaciation and Holocene in the Nordic Seas as revealed by coccolithophore assemblages
Marine Micropaleontology
(1998) - et al.
Coccolithophores in the Nordic Seascomparison of living communities with surface sediment assemblages
Deep-Sea Research II
(2000) - et al.
Coccolithophorids on the continental slope of the Bay of Biscay—production, transport and contribution to mass fluxes
Deep-Sea Research II
(1999) - et al.
Upper layer cooling and freshening in the Norwegian Sea in relation to atmospheric forcing
Deep-Sea Research I
(2000) - et al.
Coccolithus pelagicus, a productivity proxy related to moderate fronts off Western Iberia
Marine Micropaleontology
(2000) Morphology and microhabitat preferences of benthic foraminifera from the northwest Atlantic Ocean
Marine Micropaleontology
(1991)- et al.
The “Great Salinity Anomaly” in the northern North Atlantic 1968–1982
Progress in Oceanography
(1988)